Opt into PEP 517 for pip installs to avoid implicit dependency on Setuptools...

Opt into PEP 517 for pip installs to avoid implicit dependency on Setuptools to install test dependencies.

tox.ini

@@ -18,6 +18,7 @@ install_command = {[helpers]pip} install {opts} {packages}
 list_dependencies_command = {[helpers]pip} freeze --all
 setenv =
 	COVERAGE_FILE={toxworkdir}/.coverage.{envname}
+	PIP_USE_PEP517=1
 # TODO: The passed environment variables came from copying other tox.ini files
 # These should probably be individually annotated to explain what needs them.
 passenv=APPDATA HOMEDRIVE HOMEPATH windir APPVEYOR APPVEYOR_* CI CODECOV_* TRAVIS TRAVIS_* NETWORK_REQUIRED